Apparatus for the production of reflection reducing coatings



mentor G. L. DIMMICK Filed Dec. 14, 1942 APPARATUS FOR THE PRODUCTION OFREFLECTION REDUCING COATINGS Nov. 26, 1946.

@Nomen Patented Nov. 26, 1946 ENTOFFICB APPARATUS Foa THE PRODUCTION-F-REFLEo'rroN REDUCING coA'rrNGs Glenn L. Dimmick, Indianapolis, 1nd.,assigner to Radio Corporation of Delaware America, a corporation yofApplication December 14, 1342, serai-"No.sissis 3 claims. l (orsi-12.2)

This invention relates to an apparatus for producing evaporated coatingssuch,l for example, as!

reflection reducing coatings or coatings of evaporated meta-1 on opticalelements. The production of evaporated reectionreducing coatings is de`scribed, for example, in Cartwright et al. Patent No. 2,207,656. In theactual production of such4 coatings many dimculties are encountered. Oneof the difficulties is due to the fact that the coating material whenevaporatedy in vacuum either from a coiled filament or from anevaporating boat travels uniformly in alldirectons like radiant energy.As a consequence, the quantity of coating material reaching a given areaofsurface varies approximately inversely as the square of the distanceof the surface from the evaporator. The present invention provides anapparatus for rendering the coating thickness approximately uniform overa plane surface of considerable area.

One object of the invention is to provide `an improved evaporatingapparatus for the evaporation of thin films.

Another object of the-invention is to providey 1 theevaporatingapparatus which will compensate l for the normal irregularityin distribution of evaporated material.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawing in which: A

Figure 1 shows one way in which it has heretofore been attempted tosecureuniform coatings on optical surfaces.

Figure 2 is a perspective view of my improved evaporating apparatus.

Figure 3 is a top view of the evaporators and screen from the center ofthe lens holder.`

Figure 4 is a view of thel evaporators and screen taken from the outsideedge of the lens holder on a radius whose vertical .projection passesthrough an apex of the screen i4.

tion'would result in an evaporatedlm of uniform thickness upon aspherical surface with the boat or coil at its center. In practice ritis very desirable to .arrange thel evaporating system so 'that a coatingvof' uniform thickness can `be deposited on a flat surface. In his bookProcedures in Experimental Physics (page 179), Dr.

"'John Strong shows how to obtain this condition by Placing fa largenumber of evaporators in a circle whose radius is equal to the distancefrom the evaporators to the fiat object being. coated. This has manypractical objections. First it assumes a large number of evaporatorswith identical characteristics and which will vaporize exactly the sameamount of material when heated with the same current. This is diilicultto' realize in practice. l l y In a lens coating equipment, Iit; isdesirable to coat several lenses at one time and these are often intubular mountings so that they can receive the vaporized material fromonly a limited angle. Fig. 1 shows why those lenses which are in deepmountings could not receive the same quantity of evaporated material onall portions of the lens surface. It will be apparent from this figurethat the material from the evaporating coil-I2 will be obstructed by themountings of the lenses Il so that a uniform coating will not beproduced.

The improved apparatus according to the present invention as shown inFig. 2 produces much more unifrom coatings on flat'surfaces and onlenses in deep mountings. In thearrangement illustrated, threeevaporating boats are placed in a triangular arrangement and atriangular wire screen is placed `at the proper distance above theboats. This screen obstructs` a portion of the evaporated material andpermits the rest to reach the lens plate or holder L carrying the lensesIl in the -upper end of the `bell jar. lli.v vThe evaporating boats I3are made of platinum or other appropriate material and are heated byelectric currents passed through them. The-bell jar I0 Figures is adiagram inustreting the relation between lens distance and coatingthicknesses,

and v Figure 6 is a curve showing the corrected results secured by theuse of my improved apparatus. Y

Figure 7 is a view of the evaporators and'screeny when viewed from theedge of the lens holder on a. radius whose vertical projection passesthrough the center of any side of screen I4.

When a. material is evaporated from a small boat or coil placed in avacuum, the evaporated particles travel in straight lines very much likelight rays from a luminous source. 'I'his condiis of course air .tightAand is evacuated to an appropriate degree before thecommencement ofevaporation. z

As viewed from the center of the lens fplate,

' that is, from the'plane of thelenses at a point v on the common axispassing through the `centers of the evaporatorassembly, the screen andthe lens plate, the' screen I4 and the evaporators I3 appear as shown inFig. 3. It will be apparent that at this point, which is the nearest tothe evaporators, all of the evaporators are shielded bythe screenl andonly such evaporated material can reach the surface as passes throughthe Figure 7 is a similar illustration of the viewfrom f i the edge ofthe lens holder on a radius whose vertical projection passes through thecenter of any side of the screen. In`this case, vone boat is cornpletelycovered, and the other two-are each fully exposed, producing, as before,the same totalexposed area. At any other point on the edge of thecircular lens holder, the equivalent of two .boats are uncovered.Furthermore, the compensation is constant for all points on any circlewith its center on the common axisv passing vthrough the center of thelens holder, screen and -boat assembly,` an'dfproportional to the radiusof the j `circle from which the view is taken.

Consequently, the evaporation is substantially constant appropriatelyshaped piece of magnetic material I6 counterbalanced by a membe; Il,preferably of non-magnetic material. After the optical elements Il havebeen appropriately coated on one side, the lensl plate L is rotated withthe shaft I by manipulation of a magnet outside of the bell jar so as toattract'the member I6 which is fastened to the shaft' I5 and causesrotation of the lens plate L through 180 degrees to expose the oppositeside of the lens to the evaporators.

Ijclaim as my invention:

1.VV In a vacuum evaporating device for producing thin lms on thesurface oi an object, in

1 combination, a holder for said object, three elongated evaporatingelements equidistant from and in opposed relation to said holder andarranged symmetrically along the sides of an equilateral triangle, aforaminous screen in theIorm o1' an equilateral triangle `placed betweenthe evaporating elements and said holder for procuring uniform thicknessof the'evaporated material over over the whole area within the limits ofthe lens'v holder. y n

In Fig.v 5 there is diagrammatically illustrated an evaporating source Splaced at a distance h below the lens plate'L having a radius R. If bVis the radius of a'point P1 on the lens plate, the

thickness of the evaporated film at thispoint is u h2 .cos2 r2 lwhere Tois the corresponding thickness at point Po.

Fig. 6 shows how the lm thickness decreases as the distance b (from thecenter of the plate) is increased.

As an example, if we make h=6" and R=21/2" then the lm thickness at theedge of the plate would be 72.6 percent of the Lthickness at thecenterf' TheA method of compensation described Aabove is linear with b(Fig.` 5) but the attenuation in iilm thickness is not linear.

curvek (a) (Fig. 6)` shows the actuai attenuation While line (c) shows aclose approximation to this. Since only two of the boats areuncoveredwhen viewed from the edge ofthe plate as vshown inFigs. 4 or '7, thetransmission of the compensating screen would have to be 72.6X66.6:481/2 percent. A triangular piece of '-'No. 40 mesh brass screenmay be usedvfor this purpose. Its size was determined from thearrangement shown in Fig. 2, the size of the screen being determinedfrom It will be apparent to those skilled the whole surface of saidobject, said evaporating elements being at an angle to the sides of saidscreen', the position and'size oi' said screen kbeing so selected thatsaid evaporating elements are just covered by said screen when viewedfrom the center of saidholder on the common axis of said holder, screenand evaporator assembly.

2. In a vacuum vevaporating apparatus for producing thin viilms on thesurface of an object, a

alternately expose opposite sides of said objects to said evaporatingmeans, and a foraminous screen in the lform ofl an equilateral trianglemounted-between said evaporating elements and said holding means andpositioned in such a manner that said evaporating elements are at anangle to the sides of said screen and sothatsaid screen just covers saidelements when' viewed from the center of said holding means.

3. In a vacuum evaporatingapparatus for depositing a thin film on thesurface of one or more objects, a container adapted to be evacuated,three elongated evaporating boats-lying in a plane and arrangedsymmetrically along the vsides of an equilateral triangle, a holdery forsaid objects rangement may be used with other types of evap-` .oratorssuch as wire helices, straight wire filaments or the like.

In Fig. 2, thelenses Ill are vsupported on an appropriate lens plate Lwhich is mounted on a shaft l5. shaft carries onl its 'opposite end anadapted to expose the surfaces to be treated to said evaporators andhold them ina plane parallel to the plane of said boatsl and spacedtherefrom; a foraminous screen having the form'of an equilateraltriangle', positioned between said boats and said holder, parallel tothe planeof said boats,

oriented so that said boats lie symmetrically across the corners of theprojection of said screen on the plane of said boats and at such adistance from said holder that said screen just covers said boats whenviewed from the point of intersection of the plane of said objects andan axis perpendicular to the plane of said boats andpassing through thecenter of said screen. v

` GLENN L. DIMMICK.

